In petroleum refining and in petrochemical processing, hydrocarbon conversion products often are scrubbed with caustic solution. In petrochemical processing, for example, such scrubbing removes hydrogen sulfide and carbon dioxide primarily as sodium sulfide, sodium carbonate and sodium bicarbonate, and also removes some of the higher molecular weight hydrocarbon constituents. Caustic solution can be used to remove naphthenic acids and other organic acids, as well as other sulfur compounds from cracked petroleum products and petroleum distillate. However, because caustic solutions are quite harmful to organic tissue, extreme care must be taken in the use and disposal of the spent caustic solutions to protect waterways, rivers, subterranean water formations, and the like. Such spent caustic solutions often are unsuitable for direct treatment in biological wastewater treatment plants because of such factors as high pH and incompatibly high levels of biochemical oxygen demand (BOD), chemical oxygen demand (COD), and total organic carbon (TOC).
Several methods have been proposed to dispose of spent caustic. Among these are wet air oxidation, chemical oxidation, and incineration. In each of these known processes numerous pieces of equipment are needed to complete the treatment process. Such processes are capital intensive because of the number of separate process vessels involved. Likewise, such processes require a large footprint of real estate to accommodate the numerous separate pieces of equipment. Our invention has solved these problems by performing the treatment process in a single vertical column that is divided into at least three process zones. Such an apparatus and associated process represents an extremely economical method of treating waste streams, in particular spent caustic, and consequently, minimizing capital and operating costs. These and other advantages will become evident from the following more detailed description of the invention.